1. Field of the Invention
This invention relates to soldering and, in particular, to processes involving soldering with a flux.
2. Art Background
The manufacture of a wide variety of electronic devices involve solder connection induced between small regions of solder generally denominated solder bumps. For example, solder bumps present on an integrated circuit chip are aligned with corresponding solder bumps on a substrate, e.g. on a second chip, on a ceramic substrate, or on a printed circuit board. The bumps, after alignment, are held temporarily in place by procedures such as compression bonding or through the use of an adhesive such as an epoxy adhesive. (See Bartschat, Electronic Components Conference, Los Angeles, p. 335, May 1988 for a description of compression bonding.) The flux is then introduced generally at room temperature. The flux includes a vehicle and a material suitable for dissolving or removing the oxides present on the solder bumps. The assembly is then brought to the reflow temperature, i.e., a temperature sufficient to melt the solder alloy and allow it to wet the metal surfaces contacting it, and the aligned bumps are allowed to coalesce. For devices where reliability is a significant consideration, any residue present from the flux after reflow is then removed.
A variety of methods for flux introduction are available for soldering conventional assemblies. For example, as described in D. Shoenthaler, Welding Journal Research Supplement, November 1974, the assembly is immersed in a flux bath heated to the reflow temperature. As a result, the flux is both introduced and provides the heat necessary for reflow. Alternatively, as described in N. G. Koopman, et al. in Microelectronic Packaging Handbook, R. R. Tummala and E. J. Rymaszewski eds Van Nostrand Reinhold, New York, N.Y. 1989, p. 380 and references cited therein, a relatively small amount of an essentially solid flux is introduced at the solder bump region and the entire assembly brought, for example, in a furnace, to reflow temperature.
In sophisticated electronic device configurations, these conventional processes have been considered not totally desirable. For example, for many assemblies a set of solder bump connections are made between two components and these combined components are then, through a second set of solder bump connections, soldered to a third component. Clearly, the first soldering procedure must include solder bumps that melt at a temperature significantly higher than that employed during the process of soldering the combination to the third component. Typically, the assembly is made by soldering the first interconnect at temperatures in excess of 240.degree. C., and subsequently, soldering of the combination to a third component at approximately 180.degree. C. Since most fluxes, such as rosin fluxes, decompose at the temperature required for the first soldering step, this first soldering step must be done in an inert ambient, e.g., a nitrogen ambient. However, use of such ambient generally does not prevent polymeric residues to remain after soldering and produces enormous cleaning problems.
Additionally, components are now often closely positioned so that the aspect ratio between components, i.e., the ratio between the distance from the edge of the component to the furthest removed boundary of solder bump divided by the smallest distance between the two components (respectively D and d in the Figure) is larger than 0.05. The length, i.e., 10 in FIG. 1, is also often quite large, exceeding 1.5 mm and the height, 20, is quite small, less than 150 .mu.m. For such aspect ratios, heights and/or lengths, and particularly for aspect ratio greater than 0.1 and especially greater than 0.5, removal of flux residue is extremely difficult, and conventional methods are often considered inadequate for this task.
For high aspect ratios, for large lengths, and low heights as well as for high temperature processing, conventional fluxes presents significant difficulties in introducing and removing the flux and its residue. Similarly, the use of a liquid flux bath at high temperatures present significant fire hazards. These residues associated with conventional procedures are considered unacceptable risks to the lifetime of the device and interfere with subsequent coating and encapsulation. Thus, current procedures present processing shortcomings.